Study of precursor chemistry and solvent systems in pp-MOCVD processing with alumina case study

Alumina synthesis presents an opportunity to study the processing science of direct liquid injection pulsed-pressure MOCVD. The current study uses three alumina precursors and two solvent systems to study the physical chemistry of the flash evaporation processes that can occur in the pulsed-pressure approach from fundamental considerations and experimental observation. Three precursors were studied, aluminum tri-isopropoxide, aluminum tri-secbutoxide, and aluminum tri-tertbutoxide, with hexane and toluene solvents. Scoping studies identified the deposition temperature range for each precursor using silicon substrates. All deposited materials were amorphous alumina with the characteristic spheroidal microstructure. The morphology of the surfaces was used to provide the evidence to study the degree of flash evaporation, and the possible arrival of liquid droplets or solid aerosol particles. The morphological evidence consists of SEM images showing uniform coatings deposited from vapor arrival, and clusters about the size of the expected aerosols formed when droplets freeze or precursor is concentrated and dried out after flash evaporation of the solvent. The volatility of the solvent and precursor is less important for determining what degree of flash evaporation or aerosol deposition occurs than the relative diffusivity of the precursor compared to the solvent.